DeskClock: Replace ViewPager with manual fragment handling

Change-Id: Iaadfb417f0ca8638936875113ec2f39853f2a39c

14 files changed, 213 insertions, 334 deletions

diff --git a/Android.bp b/Android.bp
index 2d27ae94a..0e49ab5f1 100644
--- a/Android.bp
+++ b/Android.bp
@@ -3,6 +3,9 @@ android_app {
     resource_dirs: ["res"],
     sdk_version: "current",
     overrides: ["AlarmClock"],
+    optimize: {
+        proguard_flags_files: ["proguard.flags"],
+    },
     srcs: [
         "src/**/*.java",
         "gen/**/*.java",
diff --git a/proguard.flags b/proguard.flags
new file mode 100644
index 000000000..a6832665d
--- /dev/null
+++ b/proguard.flags
@@ -0,0 +1 @@
+-keep class androidx.viewpager.widget.* { *; }
diff --git a/res/layout/clock_fragment.xml b/res/layout/clock_fragment.xml
index ec97381a4..d1b7619f2 100644
--- a/res/layout/clock_fragment.xml
+++ b/res/layout/clock_fragment.xml
@@ -18,9 +18,8 @@
 <androidx.recyclerview.widget.RecyclerView
     xmlns:android="http://schemas.android.com/apk/res/android"
     android:id="@+id/cities"
-    android:layout_width="0dp"
+    android:layout_width="match_parent"
     android:layout_height="match_parent"
-    android:layout_weight="1"
     android:clickable="false"
     android:clipToPadding="false"
     android:paddingBottom="@dimen/fab_height"
diff --git a/res/layout/desk_clock.xml b/res/layout/desk_clock.xml
index 76ea224c3..e39d1bbbd 100644
--- a/res/layout/desk_clock.xml
+++ b/res/layout/desk_clock.xml
@@ -25,25 +25,19 @@
     app:statusBarBackground="@null"
     android:fitsSystemWindows="true">
 
-    <androidx.coordinatorlayout.widget.CoordinatorLayout
-        android:id="@+id/content"
+    <com.google.android.material.appbar.AppBarLayout
         android:layout_width="match_parent"
-        android:layout_height="0dp"
-        android:layout_weight="1">
+        android:layout_height="wrap_content"
+        android:background="@null"
+        app:elevation="0dp">
 
-        <com.google.android.material.appbar.AppBarLayout
+        <androidx.appcompat.widget.Toolbar
+            android:id="@+id/toolbar"
             android:layout_width="match_parent"
             android:layout_height="wrap_content"
-            android:background="@null"
-            app:elevation="0dp">
-
-            <androidx.appcompat.widget.Toolbar
-                android:id="@+id/toolbar"
-                android:layout_width="match_parent"
-                android:layout_height="wrap_content"
-                app:contentInsetStart="0dp"
-                tools:ignore="RtlSymmetry"
-                android:gravity="center">
+            app:contentInsetStart="0dp"
+            tools:ignore="RtlSymmetry"
+            android:gravity="center">
 
             <TextView
                 android:id="@+id/title_view"
@@ -52,16 +46,19 @@
                 android:layout_gravity="center"
                 style="@style/TextAppearance.AppCompat.Widget.ActionBar.Title" />
 
-            </androidx.appcompat.widget.Toolbar>
-        </com.google.android.material.appbar.AppBarLayout>
+        </androidx.appcompat.widget.Toolbar>
+    </com.google.android.material.appbar.AppBarLayout>
+
+    <androidx.coordinatorlayout.widget.CoordinatorLayout
+        android:id="@+id/content"
+        android:layout_width="match_parent"
+        android:layout_height="0dp"
+        android:layout_weight="1">
 
-        <androidx.viewpager.widget.ViewPager
-            android:id="@+id/desk_clock_pager"
+        <FrameLayout
+            android:id="@+id/fragment_container"
             android:layout_width="match_parent"
-            android:layout_height="match_parent"
-            android:importantForAccessibility="no"
-            android:saveEnabled="false"
-            app:layout_behavior="@string/appbar_scrolling_view_behavior" />
+            android:layout_height="match_parent"/>
 
         <LinearLayout
             android:layout_width="match_parent"
diff --git a/src/com/android/deskclock/AlarmClockFragment.java b/src/com/android/deskclock/AlarmClockFragment.java
index ac1966a0d..70ffee3a5 100644
--- a/src/com/android/deskclock/AlarmClockFragment.java
+++ b/src/com/android/deskclock/AlarmClockFragment.java
@@ -31,6 +31,7 @@ import android.widget.TextView;
 import androidx.annotation.NonNull;
 import androidx.loader.app.LoaderManager;
 import androidx.loader.content.Loader;
+import androidx.loader.content.CursorLoader;
 import androidx.recyclerview.widget.LinearLayoutManager;
 import androidx.recyclerview.widget.RecyclerView;
 
@@ -81,7 +82,7 @@ public final class AlarmClockFragment extends DeskClockFragment implements
     private RecyclerView mRecyclerView;
 
     // Data
-    private Loader mCursorLoader;
+    private CursorLoader mCursorLoader;
     private long mScrollToAlarmId = Alarm.INVALID_ID;
     private long mExpandedAlarmId = Alarm.INVALID_ID;
     private long mCurrentUpdateToken;
@@ -103,7 +104,7 @@ public final class AlarmClockFragment extends DeskClockFragment implements
     @Override
     public void onCreate(Bundle savedState) {
         super.onCreate(savedState);
-        mCursorLoader = LoaderManager.getInstance(this).initLoader(0, null, this);
+        mCursorLoader = (CursorLoader) LoaderManager.getInstance(this).initLoader(0, null, this);
         if (savedState != null) {
             mExpandedAlarmId = savedState.getLong(KEY_EXPANDED_ID, Alarm.INVALID_ID);
         }
@@ -402,6 +403,11 @@ public final class AlarmClockFragment extends DeskClockFragment implements
         right.setVisibility(View.INVISIBLE);
     }
 
+    @Override
+    public final int getFabTargetVisibility() {
+        return View.VISIBLE;
+    }
+
     private void startCreatingAlarm() {
         // Clear the currently selected alarm.
         mAlarmTimeClickHandler.setSelectedAlarm(null);
diff --git a/src/com/android/deskclock/ClockFragment.java b/src/com/android/deskclock/ClockFragment.java
index 1536b55df..b487e52be 100644
--- a/src/com/android/deskclock/ClockFragment.java
+++ b/src/com/android/deskclock/ClockFragment.java
@@ -215,6 +215,11 @@ public final class ClockFragment extends DeskClockFragment {
         right.setVisibility(INVISIBLE);
     }
 
+    @Override
+    public final int getFabTargetVisibility() {
+        return View.VISIBLE;
+    }
+
     /**
      * Refresh the next alarm time.
      */
diff --git a/src/com/android/deskclock/DeskClock.java b/src/com/android/deskclock/DeskClock.java
index cc3687002..4dea5d269 100644
--- a/src/com/android/deskclock/DeskClock.java
+++ b/src/com/android/deskclock/DeskClock.java
@@ -88,9 +88,6 @@ public class DeskClock extends BaseActivity
     /** Hides, updates, and shows only the {@link #mLeftButton} and {@link #mRightButton}. */
     private final AnimatorSet mUpdateButtonsOnlyAnimation = new AnimatorSet();
 
-    /** Automatically starts the {@link #mShowAnimation} after {@link #mHideAnimation} ends. */
-    private final AnimatorListenerAdapter mAutoStartShowListener = new AutoStartShowListener();
-
     /** Updates the user interface to reflect the selected tab from the backing model. */
     private final TabListener mTabChangeWatcher = new TabChangeWatcher();
 
@@ -119,15 +116,14 @@ public class DeskClock extends BaseActivity
     /** The ViewPager that pages through the fragments representing the content of the tabs. */
     private ViewPager mFragmentTabPager;
 
-    /** Generates the fragments that are displayed by the {@link #mFragmentTabPager}. */
-    private FragmentTabPagerAdapter mFragmentTabPagerAdapter;
-
     /** The view that displays the current tab's title */
     private TextView mTitleView;
 
     /** The bottom navigation bar */
     private BottomNavigationView mBottomNavigation;
 
+    private FragmentUtils mFragmentUtils;
+
     /** {@code true} when a settings change necessitates recreating this activity. */
     private boolean mRecreateActivity;
 
@@ -248,18 +244,7 @@ public class DeskClock extends BaseActivity
                 .after(leftHideAnimation)
                 .after(rightHideAnimation);
 
-        // Customize the view pager.
-        mFragmentTabPagerAdapter = new FragmentTabPagerAdapter(this);
-        mFragmentTabPager = (ViewPager) findViewById(R.id.desk_clock_pager);
-        // Keep all four tabs to minimize jank.
-        mFragmentTabPager.setOffscreenPageLimit(3);
-        // Set Accessibility Delegate to null so view pager doesn't intercept movements and
-        // prevent the fab from being selected.
-        mFragmentTabPager.setAccessibilityDelegate(null);
-        // Mirror changes made to the selected page of the view pager into UiDataModel.
-        mFragmentTabPager.addOnPageChangeListener(new PageChangeWatcher());
-        mFragmentTabPager.setAdapter(mFragmentTabPagerAdapter);
-
+        mFragmentUtils = new FragmentUtils(this);
         // Mirror changes made to the selected tab into UiDataModel.
         mBottomNavigation = findViewById(R.id.bottom_view);
         mBottomNavigation.setOnNavigationItemSelectedListener(mNavigationListener);
@@ -275,27 +260,40 @@ public class DeskClock extends BaseActivity
 
         @Override
         public boolean onNavigationItemSelected(@NonNull MenuItem item) {
-            UiDataModel.Tab tab = null;
+            UiDataModel.Tab selectedTab = null;
             switch (item.getItemId()) {
                 case R.id.page_alarm:
-                    tab = UiDataModel.Tab.ALARMS;
+                    selectedTab = UiDataModel.Tab.ALARMS;
                     break;
 
                 case R.id.page_clock:
-                    tab = UiDataModel.Tab.CLOCKS;
+                    selectedTab = UiDataModel.Tab.CLOCKS;
                     break;
 
                 case R.id.page_timer:
-                    tab = UiDataModel.Tab.TIMERS;
+                    selectedTab = UiDataModel.Tab.TIMERS;
                     break;
 
                 case R.id.page_stopwatch:
-                    tab = UiDataModel.Tab.STOPWATCH;
+                    selectedTab = UiDataModel.Tab.STOPWATCH;
                     break;
             }
 
-            if (tab != null) {
-                UiDataModel.getUiDataModel().setSelectedTab(tab);
+            if (selectedTab != null) {
+                UiDataModel.Tab currentTab = UiDataModel.getUiDataModel().getSelectedTab();
+                DeskClockFragment currentFrag = mFragmentUtils.getDeskClockFragment(currentTab);
+                DeskClockFragment selectedFrag = mFragmentUtils.getDeskClockFragment(selectedTab);
+
+                int currentVisibility = currentFrag.getFabTargetVisibility();
+                int targetVisibility = selectedFrag.getFabTargetVisibility();
+                if (currentVisibility != targetVisibility) {
+                    if (targetVisibility == View.VISIBLE) {
+                        mShowAnimation.start();
+                    } else {
+                        mHideAnimation.start();
+                    }
+                }
+                UiDataModel.getUiDataModel().setSelectedTab(selectedTab);
                 return true;
             }
 
@@ -324,15 +322,6 @@ public class DeskClock extends BaseActivity
 
         if (mRecreateActivity) {
             mRecreateActivity = false;
-
-            // A runnable must be posted here or the new DeskClock activity will be recreated in a
-            // paused state, even though it is the foreground activity.
-            mFragmentTabPager.post(new Runnable() {
-                @Override
-                public void run() {
-                    recreate();
-                }
-            });
         }
     }
 
@@ -468,16 +457,7 @@ public class DeskClock extends BaseActivity
         final UiDataModel.Tab selectedTab = UiDataModel.getUiDataModel().getSelectedTab();
         // Update the selected tab in the mBottomNavigation if it does not agree with UiDataModel.
         mBottomNavigation.setSelectedItemId(selectedTab.getPageResId());
-
-        // Update the selected fragment in the viewpager if it does not agree with UiDataModel.
-        for (int i = 0; i < mFragmentTabPagerAdapter.getCount(); i++) {
-            final DeskClockFragment fragment = mFragmentTabPagerAdapter.getDeskClockFragment(i);
-            if (fragment.isTabSelected() && mFragmentTabPager.getCurrentItem() != i) {
-                mFragmentTabPager.setCurrentItem(i);
-                break;
-            }
-        }
-
+        mFragmentUtils.showFragment(selectedTab);
         mTitleView.setText(selectedTab.getLabelResId());
     }
 
@@ -485,14 +465,7 @@ public class DeskClock extends BaseActivity
      * @return the DeskClockFragment that is currently selected according to UiDataModel
      */
     private DeskClockFragment getSelectedDeskClockFragment() {
-        for (int i = 0; i < mFragmentTabPagerAdapter.getCount(); i++) {
-            final DeskClockFragment fragment = mFragmentTabPagerAdapter.getDeskClockFragment(i);
-            if (fragment.isTabSelected()) {
-                return fragment;
-            }
-        }
-        final UiDataModel.Tab selectedTab = UiDataModel.getUiDataModel().getSelectedTab();
-        throw new IllegalStateException("Unable to locate selected fragment (" + selectedTab + ")");
+        return mFragmentUtils.getCurrentFragment();
     }
 
     /**
@@ -503,93 +476,6 @@ public class DeskClock extends BaseActivity
     }
 
     /**
-     * As the view pager changes the selected page, update the model to record the new selected tab.
-     */
-    private final class PageChangeWatcher implements OnPageChangeListener {
-
-        /** The last reported page scroll state; used to detect exotic state changes. */
-        private int mPriorState = SCROLL_STATE_IDLE;
-
-        public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
-            // Only hide the fab when a non-zero drag distance is detected. This prevents
-            // over-scrolling from needlessly hiding the fab.
-            if (mFabState == FabState.HIDE_ARMED && positionOffsetPixels != 0) {
-                mFabState = FabState.HIDING;
-                mHideAnimation.start();
-            }
-        }
-
-        @Override
-        public void onPageScrollStateChanged(int state) {
-            if (mPriorState == SCROLL_STATE_IDLE && state == SCROLL_STATE_SETTLING) {
-                // The user has tapped a tab button; play the hide and show animations linearly.
-                mHideAnimation.addListener(mAutoStartShowListener);
-                mHideAnimation.start();
-                mFabState = FabState.HIDING;
-            } else if (mPriorState == SCROLL_STATE_SETTLING && state == SCROLL_STATE_DRAGGING) {
-                // The user has interrupted settling on a tab and the fab button must be re-hidden.
-                if (mShowAnimation.isStarted()) {
-                    mShowAnimation.cancel();
-                }
-                if (mHideAnimation.isStarted()) {
-                    // Let the hide animation finish naturally; don't auto show when it ends.
-                    mHideAnimation.removeListener(mAutoStartShowListener);
-                } else {
-                    // Start and immediately end the hide animation to jump to the hidden state.
-                    mHideAnimation.start();
-                    mHideAnimation.end();
-                }
-                mFabState = FabState.HIDING;
-
-            } else if (state != SCROLL_STATE_DRAGGING && mFabState == FabState.HIDING) {
-                // The user has lifted their finger; show the buttons now or after hide ends.
-                if (mHideAnimation.isStarted()) {
-                    // Finish the hide animation and then start the show animation.
-                    mHideAnimation.addListener(mAutoStartShowListener);
-                } else {
-                    updateFab(FAB_AND_BUTTONS_IMMEDIATE);
-                    mShowAnimation.start();
-
-                    // The animation to show the fab has begun; update the state to showing.
-                    mFabState = FabState.SHOWING;
-                }
-            } else if (state == SCROLL_STATE_DRAGGING) {
-                // The user has started a drag so arm the hide animation.
-                mFabState = FabState.HIDE_ARMED;
-            }
-
-            // Update the last known state.
-            mPriorState = state;
-        }
-
-        @Override
-        public void onPageSelected(int position) {
-            mFragmentTabPagerAdapter.getDeskClockFragment(position).selectTab();
-        }
-    }
-
-    /**
-     * If this listener is attached to {@link #mHideAnimation} when it ends, the corresponding
-     * {@link #mShowAnimation} is automatically started.
-     */
-    private final class AutoStartShowListener extends AnimatorListenerAdapter {
-        @Override
-        public void onAnimationEnd(Animator animation) {
-            // Prepare the hide animation for its next use; by default do not auto-show after hide.
-            mHideAnimation.removeListener(mAutoStartShowListener);
-
-            // Update the buttons now that they are no longer visible.
-            updateFab(FAB_AND_BUTTONS_IMMEDIATE);
-
-            // Automatically start the grow animation now that shrinking is complete.
-            mShowAnimation.start();
-
-            // The animation to show the fab has begun; update the state to showing.
-            mFabState = FabState.SHOWING;
-        }
-    }
-
-    /**
      * Shows/hides a snackbar as silencing settings are enabled/disabled.
      */
     private final class SilentSettingChangeWatcher implements OnSilentSettingsListener {
@@ -666,6 +552,7 @@ public class DeskClock extends BaseActivity
             // If the hide animation has already completed, the buttons must be updated now when the
             // new tab is known. Otherwise they are updated at the end of the hide animation.
             if (!mHideAnimation.isStarted()) {
+                getSupportFragmentManager().executePendingTransactions();
                 updateFab(FAB_AND_BUTTONS_IMMEDIATE);
             }
         }
diff --git a/src/com/android/deskclock/DeskClockFragment.java b/src/com/android/deskclock/DeskClockFragment.java
index 1f51a332e..c08a15a8e 100644
--- a/src/com/android/deskclock/DeskClockFragment.java
+++ b/src/com/android/deskclock/DeskClockFragment.java
@@ -20,6 +20,7 @@ import android.view.KeyEvent;
 import android.widget.Button;
 import android.widget.ImageView;
 import androidx.annotation.ColorInt;
+import androidx.annotation.Keep;
 import androidx.annotation.NonNull;
 import androidx.fragment.app.Fragment;
 
@@ -104,6 +105,7 @@ public abstract class DeskClockFragment extends Fragment implements FabContainer
     /**
      * Select the tab that displays this fragment.
      */
+    @Keep
     public final void selectTab() {
         UiDataModel.getUiDataModel().setSelectedTab(mTab);
     }
diff --git a/src/com/android/deskclock/FabController.java b/src/com/android/deskclock/FabController.java
index bab7f46ad..11c5d5375 100644
--- a/src/com/android/deskclock/FabController.java
+++ b/src/com/android/deskclock/FabController.java
@@ -57,4 +57,10 @@ public interface FabController {
      * @param right the button to the right of the fab component
      */
     void onRightButtonClick(@NonNull Button right);
-}
\ No newline at end of file
+
+    /**
+     *
+     * @return the target visibility of the FAB component
+     */
+    int getFabTargetVisibility();
+}
diff --git a/src/com/android/deskclock/FragmentTabPagerAdapter.java b/src/com/android/deskclock/FragmentTabPagerAdapter.java
deleted file mode 100644
index de3e25c35..000000000
--- a/src/com/android/deskclock/FragmentTabPagerAdapter.java
+++ /dev/null
@@ -1,167 +0,0 @@
-/*
- * Copyright (C) 2016 The Android Open Source Project
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- *      http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-
-package com.android.deskclock;
-
-import android.util.ArrayMap;
-import android.view.View;
-import android.view.ViewGroup;
-import androidx.fragment.app.Fragment;
-import androidx.fragment.app.FragmentManager;
-import androidx.fragment.app.FragmentTransaction;
-import androidx.viewpager.widget.PagerAdapter;
-
-import com.android.deskclock.uidata.UiDataModel;
-
-import java.util.Map;
-
-/**
- * This adapter produces the DeskClockFragments that are the content of the DeskClock tabs. The
- * adapter presents the tabs in LTR and RTL order depending on the text layout direction for the
- * current locale. To prevent issues when switching between LTR and RTL, fragments are registered
- * with the manager using position-independent tags, which is an important departure from
- * FragmentPagerAdapter.
- */
-final class FragmentTabPagerAdapter extends PagerAdapter {
-
-    private final DeskClock mDeskClock;
-
-    /** The manager into which fragments are added. */
-    private final FragmentManager mFragmentManager;
-
-    /** A fragment cache that can be accessed before {@link #instantiateItem} is called. */
-    private final Map<UiDataModel.Tab, DeskClockFragment> mFragmentCache;
-
-    /** The active fragment transaction if one exists. */
-    private FragmentTransaction mCurrentTransaction;
-
-    /** The current fragment displayed to the user. */
-    private Fragment mCurrentPrimaryItem;
-
-    FragmentTabPagerAdapter(DeskClock deskClock) {
-        mDeskClock = deskClock;
-        mFragmentCache = new ArrayMap<>(getCount());
-        mFragmentManager = deskClock.getSupportFragmentManager();
-    }
-
-    @Override
-    public int getCount() {
-        return UiDataModel.getUiDataModel().getTabCount();
-    }
-
-    /**
-     * @param position the left-to-right index of the fragment to be returned
-     * @return the fragment displayed at the given {@code position}
-     */
-    DeskClockFragment getDeskClockFragment(int position) {
-        // Fetch the tab the UiDataModel reports for the position.
-        final UiDataModel.Tab tab = UiDataModel.getUiDataModel().getTabAt(position);
-
-        // First check the local cache for the fragment.
-        DeskClockFragment fragment = mFragmentCache.get(tab);
-        if (fragment != null) {
-            return fragment;
-        }
-
-        // Next check the fragment manager; relevant when app is rebuilt after locale changes
-        // because this adapter will be new and mFragmentCache will be empty, but the fragment
-        // manager will retain the Fragments built on original application launch.
-        fragment = (DeskClockFragment) mFragmentManager.findFragmentByTag(tab.name());
-        if (fragment != null) {
-            fragment.setFabContainer(mDeskClock);
-            mFragmentCache.put(tab, fragment);
-            return fragment;
-        }
-
-        // Otherwise, build the fragment from scratch.
-        final String fragmentClassName = tab.getFragmentClassName();
-        fragment = (DeskClockFragment) Fragment.instantiate(mDeskClock, fragmentClassName);
-        fragment.setFabContainer(mDeskClock);
-        mFragmentCache.put(tab, fragment);
-        return fragment;
-    }
-
-    @Override
-    public void startUpdate(ViewGroup container) {
-        if (container.getId() == View.NO_ID) {
-            throw new IllegalStateException("ViewPager with adapter " + this + " has no id");
-        }
-    }
-
-    @Override
-    public Object instantiateItem(ViewGroup container, int position) {
-        if (mCurrentTransaction == null) {
-            mCurrentTransaction = mFragmentManager.beginTransaction();
-        }
-
-        // Use the fragment located in the fragment manager if one exists.
-        final UiDataModel.Tab tab = UiDataModel.getUiDataModel().getTabAt(position);
-        Fragment fragment = mFragmentManager.findFragmentByTag(tab.name());
-        if (fragment != null) {
-            mCurrentTransaction.attach(fragment);
-        } else {
-            fragment = getDeskClockFragment(position);
-            mCurrentTransaction.add(container.getId(), fragment, tab.name());
-        }
-
-        if (fragment != mCurrentPrimaryItem) {
-            fragment.setMenuVisibility(false);
-            fragment.setUserVisibleHint(false);
-        }
-
-        return fragment;
-    }
-
-    @Override
-    public void destroyItem(ViewGroup container, int position, Object object) {
-        if (mCurrentTransaction == null) {
-            mCurrentTransaction = mFragmentManager.beginTransaction();
-        }
-        final DeskClockFragment fragment = (DeskClockFragment) object;
-        fragment.setFabContainer(null);
-        mCurrentTransaction.detach(fragment);
-    }
-
-    @Override
-    public void setPrimaryItem(ViewGroup container, int position, Object object) {
-        final Fragment fragment = (Fragment) object;
-        if (fragment != mCurrentPrimaryItem) {
-            if (mCurrentPrimaryItem != null) {
-                mCurrentPrimaryItem.setMenuVisibility(false);
-                mCurrentPrimaryItem.setUserVisibleHint(false);
-            }
-            if (fragment != null) {
-                fragment.setMenuVisibility(true);
-                fragment.setUserVisibleHint(true);
-            }
-            mCurrentPrimaryItem = fragment;
-        }
-    }
-
-    @Override
-    public void finishUpdate(ViewGroup container) {
-        if (mCurrentTransaction != null) {
-            mCurrentTransaction.commitAllowingStateLoss();
-            mCurrentTransaction = null;
-            mFragmentManager.executePendingTransactions();
-        }
-    }
-
-    @Override
-    public boolean isViewFromObject(View view, Object object) {
-        return ((Fragment) object).getView() == view;
-    }
-}
\ No newline at end of file
diff --git a/src/com/android/deskclock/FragmentUtils.java b/src/com/android/deskclock/FragmentUtils.java
new file mode 100644
index 000000000..43d7a4dc7
--- /dev/null
+++ b/src/com/android/deskclock/FragmentUtils.java
@@ -0,0 +1,113 @@
+/*
+ * Copyright (C) 2016 The Android Open Source Project
+ * Copyright (C) 2020 The LineageOS Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.android.deskclock;
+
+import android.util.ArrayMap;
+import android.view.View;
+import android.view.ViewGroup;
+import androidx.fragment.app.Fragment;
+import androidx.fragment.app.FragmentManager;
+import androidx.fragment.app.FragmentTransaction;
+
+import com.android.deskclock.uidata.UiDataModel;
+
+import java.util.Map;
+
+/**
+ * This class produces the DeskClockFragments that are the content of the DeskClock tabs.
+ * It presents the tabs in LTR and RTL order depending on the text layout direction for the
+ * current locale. To prevent issues when switching between LTR and RTL, fragments are registered
+ * with the manager using position-independent tags, which is an important departure from
+ * FragmentPagerAdapter.
+ */
+public final class FragmentUtils {
+
+    private final DeskClock mDeskClock;
+
+    /** The manager into which fragments are added. */
+    private final FragmentManager mFragmentManager;
+
+    /** A fragment cache that can be accessed before {@link #instantiateItem} is called. */
+    private final Map<UiDataModel.Tab, DeskClockFragment> mFragmentCache;
+
+    /** The current fragment displayed to the user. */
+    private DeskClockFragment mCurrentPrimaryItem;
+
+    public FragmentUtils(DeskClock deskClock) {
+        mDeskClock = deskClock;
+        mFragmentCache = new ArrayMap<>(getCount());
+        mFragmentManager = deskClock.getSupportFragmentManager();
+    }
+
+    private int getCount() {
+        return UiDataModel.getUiDataModel().getTabCount();
+    }
+
+    public DeskClockFragment getDeskClockFragment(UiDataModel.Tab tab) {
+        // First check the local cache for the fragment.
+        DeskClockFragment fragment = mFragmentCache.get(tab);
+        if (fragment != null) {
+            return fragment;
+        }
+
+        // Next check the fragment manager; relevant when app is rebuilt after locale changes
+        // because this adapter will be new and mFragmentCache will be empty, but the fragment
+        // manager will retain the Fragments built on original application launch.
+        fragment = (DeskClockFragment) mFragmentManager.findFragmentByTag(tab.name());
+        if (fragment != null) {
+            fragment.setFabContainer(mDeskClock);
+            mFragmentCache.put(tab, fragment);
+            return fragment;
+        }
+
+        // Otherwise, build the fragment from scratch.
+        final String fragmentClassName = tab.getFragmentClassName();
+        fragment = (DeskClockFragment) Fragment.instantiate(mDeskClock, fragmentClassName);
+        fragment.setFabContainer(mDeskClock);
+
+        FragmentTransaction transaction = mFragmentManager.beginTransaction();
+        transaction.add(R.id.fragment_container, fragment, tab.name());
+        transaction.commit();
+
+        mFragmentCache.put(tab, fragment);
+        return fragment;
+    }
+
+    public void hideAllFragments() {
+        FragmentTransaction transaction = mFragmentManager.beginTransaction();
+        for (UiDataModel.Tab tab : UiDataModel.Tab.values()) {
+            Fragment fragment = mFragmentManager.findFragmentByTag(tab.name());
+            if (fragment != null) {
+                transaction.hide(fragment);
+            }
+        }
+
+        transaction.commit();
+    }
+
+    public void showFragment(UiDataModel.Tab tab) {
+        hideAllFragments();
+        Fragment fragment = getDeskClockFragment(tab);
+        mFragmentManager.beginTransaction().show(fragment).commit();
+        mCurrentPrimaryItem = (DeskClockFragment) fragment;
+    }
+
+    public DeskClockFragment getCurrentFragment() {
+        return mCurrentPrimaryItem;
+    }
+}
\ No newline at end of file
diff --git a/src/com/android/deskclock/stopwatch/StopwatchFragment.java b/src/com/android/deskclock/stopwatch/StopwatchFragment.java
index d0b2466fe..87d22ea60 100644
--- a/src/com/android/deskclock/stopwatch/StopwatchFragment.java
+++ b/src/com/android/deskclock/stopwatch/StopwatchFragment.java
@@ -294,6 +294,11 @@ public final class StopwatchFragment extends DeskClockFragment {
         }
     }
 
+    @Override
+    public final int getFabTargetVisibility() {
+        return View.VISIBLE;
+    }
+
     /**
      * @param color the newly installed app window color
      */
diff --git a/src/com/android/deskclock/timer/TimerFragment.java b/src/com/android/deskclock/timer/TimerFragment.java
index 7779be656..0b2a49cec 100644
--- a/src/com/android/deskclock/timer/TimerFragment.java
+++ b/src/com/android/deskclock/timer/TimerFragment.java
@@ -236,6 +236,26 @@ public final class TimerFragment extends DeskClockFragment {
         }
     }
 
+    @Override
+    public int getFabTargetVisibility() {
+        if (mCurrentView == mTimersView) {
+            final Timer timer = getTimer();
+            if (timer == null) {
+                return INVISIBLE;
+            } else {
+                return VISIBLE;
+            }
+        } else if (mCurrentView == mCreateTimerView) {
+            if (mCreateTimerView.hasValidInput()) {
+                return VISIBLE;
+            } else {
+                return INVISIBLE;
+            }
+        }
+
+        return INVISIBLE;
+    }
+
     private void updateFab(@NonNull ImageView fab) {
         if (mCurrentView == mTimersView) {
             final Timer timer = getTimer();
diff --git a/src/com/android/deskclock/uidata/TabModel.java b/src/com/android/deskclock/uidata/TabModel.java
index 5b878ed9d..9a1af3bd0 100644
--- a/src/com/android/deskclock/uidata/TabModel.java
+++ b/src/com/android/deskclock/uidata/TabModel.java
@@ -18,6 +18,7 @@ package com.android.deskclock.uidata;
 
 import android.content.SharedPreferences;
 import android.text.TextUtils;
+import androidx.annotation.Keep;
 
 import java.util.ArrayList;
 import java.util.Arrays;
@@ -88,6 +89,7 @@ final class TabModel {
      * @param position the position of the tab in the user interface
      * @return the tab at the given {@code ordinal}
      */
+    @Keep
     Tab getTabAt(int position) {
         final int ordinal;
         if (TextUtils.getLayoutDirectionFromLocale(Locale.getDefault()) == LAYOUT_DIRECTION_RTL) {